This is a matter of sloppy execution first and foremost. Technically, not every companion is supposed to be propositioning the player at the same time, only those with high enough approval. The problem is, the required approval threshold is very low right now (Medium, only a step above Neutral, which everyone starts at) and it's very easy to get everyone to that level. Simply giving Gale two artifacts will put him way above the requirement, and the same is true for Wyll if you do his quest and Lae'zel if you let her take charge during hers. For Astarion it's enough to give him your blood, let him feed on enemies, and kill the hunter with him. Shadowheart is probably the trickiest, as she doesn't have any big moments right now, but she approves of being nice to animals, which most people will do naturally, and lying, which the players will also be doing out of necessity while exploring the goblin camp. This results in a shower of proposals at the party, and yes, it's quite jarring, even if the occasion is suitable.
Compounding the problem is the fact that there's nothing to do at the party other than pick who you (don't) want to sleep with. Every companion has a short exchange where you can either accept or reject their advances, and they just send you away after that. You can't celebrate with them or get to know them better, it's all about hooking up. The companions don't even have alternate dialogue trees if you don't have enough approval with them, they start the conversation as if they want to get with you, but backpedal at the last moment, and still act jealous if you pick someone else and talk to them afterwards. It's fine for Gale to say he's thankful for being able to spend time "with just the two of us" if he's trying to get you to bed, but why is he using such suggestive language if he's not interested? It's amusing to listen to Astarion try to insult whoever you picked, but he really shouldn't be acting so spurned if he didn't want to get into your pants in the first place. And so on for every other companion; I think only Shadowheart and Wyll act like adults if you don't pick them. This way of structuring dialogue makes it seem like everyone wants a piece of you even if they actually don't.
My hope is that this is an EA-only thing. Larian want to show off different aspects of the game, romances included, and made them so easily accessible for that reason. Hopefully, in the full game it will be harder to gain enough approval and there'll be options to celebrate the victory in other ways.
As for skipping the party, I don't think it's a problem. During it you can say "Maybe another time" to companions you didn't pick, so this obviosuly won't be the only opportunity to enter into a romance with someone. Not to mention, you're not actually starting a relationship with anyone at the party, except for maybe Gale and Shadowheart (and even with those two it's not certain), you're just spending the night together. There's not much weight to it.
